#define _CRT_SECURE_NO_WARNINGS 1
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* prev = nullptr;
    void flatten(TreeNode* root)
    {
        if (root == nullptr)
        {
            return;
        }
        cout << root->val << endl;
        TreeNode* left = root->left;
        TreeNode* right = root->right;
        if (prev != nullptr)
        {
            prev->right = root;
            prev->left = nullptr;
        }
        prev = root;
        flatten(left);
        flatten(right);
        return;
    }
};